1. Field of the Invention
The present invention relates to a method of detecting a correct potential at the end point of titration having regard to an electrode potential caused by a liquid resistance of a liquid sample in a Karl Fischer titration for determining a small amount of water contained in the liquid sample.
2. Description of the Prior Art
Karl Fischer's method has been generally known as a method of determining a small amount of water contained in solids, liquids or gases. This is a kind of moisture determination based on the fact that water reacts quantitatively with iodine and sulfur dioxide gas in the presence of methyl alcohol and pyridine in accordance with the following equations: EQU SO.sub.2 +I.sub.2 +H.sub.2 O+3C.sub.5 H.sub.5 N.fwdarw.2C.sub.5 H.sub.5 N.HI+C.sub.5 H.sub.5 N.SO.sub.3 EQU C.sub.5 H.sub.5 N.SO.sub.3 +CH.sub.3 OH.fwdarw.C.sub.5 H.sub.5 N.HSO.sub.4 CH.sub.3
The end point of titration is accurately detected by the dead stop end point method in which two pieces of platinum electrode immersed in a liquid sample are polarized by allowing a minute electric current to flow therebetween and the change of potential caused by an excess of Karl Fischer reagent is detected.
A small amount of water contained in a sample has been determined as follows on the bsis of the above described principle: Referring to FIG. 2, numeral 1 designates a titration flask for use in the conventional Karl Fischer moisture meter in which a liquid sample (organic solvent+sample) is put and the titration is carried out by running Karl Fischer reagent thereinto. The titration flask 1 is provided with an injection port 2 of Karl Fischer reagent and two pieces of platinum electrode 3 for detecting the end point of titration, the opening port being closed with a rubber stopper 4. An appointed quantity of a liquid sample is injected into the titration flask 1 through a syringe 5, of which pointed end needle portion runs through the rubber stopper 4, so as not to bring the liquid sample into contact with the atmospheric moisture, preventing any change in quantity of water in the liquid sample.
At first, a direct current signal, a low-frequency alternating current signal or a pulse signal is applied to the platinum electrodes 3 and an electric potential, which is produced between the electrodes 3, is measured. When the water contained in the liquid sample is in excess, the polarization potential of the platinum electrodes 3 is raised (polarized state), resulting in a poor flow of an electric current. Then, Karl Fischer reagent gradually becomes excessive with the consumption of water as a result of the titration, whereby the polarization potential of the platinum electrodes 3 is lowered (depolarized state), and an electric current flows easily. That is to say, the water is gradually consumed with the titration until it is exhausted, whereupon an electrode potential undergoes a sudden change. The concentration of water contained in the liquid sample can be determined from the quantity of Karl Fischer reagent used for the titration up until the sudden change of the electrode potential takes place. The titration curve showing the electrode potential plotted against the concentration of Karl Fischer reagent is drawn with the dotted curve C.sub.1 in FIG. 3. In the titration, it is only necessary to presume the curve C.sub.1, set the end point potential V.sub.1, and run Karl Fischer reagent into the liquid sample until the end point potential V.sub.1 is reached.
An electrode potential of the platinum electrodes 3 not only undergoes a change at the time when the liquid sample turns from a water-excesss state to a Karl Fischer reagent-excess state but also depends upon the liquid resistance of the liquid sample. Assuming that the liquid resistance of the liquid sample is a constant and gives rise to a voltage drop V.sub.2 as shown in FIG. 3, an electrode potential does not become lower than V.sub.2 and the titration curve, curve C.sub.2 in this case, lies above a straight line V.sub.2. As a result, even though Karl Fischer reagent becomes excessive in the liquid sample, an electrode potential does not come down to the end point potential V.sub.1, whereby the determination becomes impossible. Therefore, measures such as the adjustment of the setting of the end point potential or the selection of lower resistant organic solvents as a constituent of the liquid sample can be taken. But, both of them are unsatisfactory from the standpoint of operability in that the adjustment or the selection should be made for every sample. In addition, even though the setting of the end point potential V.sub.1 is adjusted as above described, the liquid resistance of the liquid sample can change in the course of titration. For example, where the voltage caused by the liquid resistance varies dependent upon the concentration of Karl Fischer reagent--that is, as the titration progresses--the voltage curve becomes that illustrated by the curve V.sub.3 in FIG. 4. The titration curve showing the change of an electrode potential is the curve C.sub.3, and the curve obtained by subtracting the voltage V.sub.3 from an electrode potential, that is the curve C.sub.1, is the original titration curve.
Accordingly, the concentration D.sub.3 of Karl Fischer reagent at the end point potential V.sub.1 obtained from the curve C.sub.3 deviates from the original concentration D.sub.1 of Karl Fischer reagent at the end point potential V.sub.1 obtained from the curve C.sub.1, leading to an error in the result.